In conventional thin film-type electroluminescent (EL) devices, an inorganic material group II-VI compound semiconductor such as ZnS, CaS and SrS is generally doped with Mn or a rare earth element (e.g., Eu, Ce, Th, Sm) as a emission center. However, the EL device fabricated from the above-described inorganic material has problems such that:                1) a.c. (alternating current) driving is necessary (from 50 to 1,000 Hz),        2) the driving voltage is high (up to 200 V),        3) full color emission is difficult (particularly blue) and        4) the peripheral driving circuit costs highly.        
In recent years, however, for the purpose of improving the above-described problems, studies have been made to develop an EL device using an organic thin film. In order to elevate the light emission efficiency, an organic electroluminescent device has been developed, where the kind of electrode is optimized for the purpose of improving the efficiency of carrier injection from an electrode and a hole-transporting layer comprising an aromatic diamine and a light-emitting layer comprising an aluminum complex of 8-hydroxyquinoline are provide (see, Appl. Phys. Lett., Vol. 51, page 913 (1987)). By this technique, the light emission efficiency is greatly improved as compared with conventional EL devices using a single crystal such as anthracene. Furthermore, for example, an aluminum complex of 8-hydroxyquinoline as a host material is doped with a fluorescence dye for laser, such as coumarin (see, J. Appl. Phys., Vol. 65, page 3610 (1989)), with an attempt to improve the light emission efficiency, convert the light emission wavelength or the like. The devices are coming to have practically usable properties.
In addition to these electroluminescent devices using a low-molecule-weight material, studies are being made to develop an electroluminescent device using a polymer material such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] or poly(3-alkyl-thiophene) for the light-emitting layer material or to develop a device using a low molecule light-emitting material and an electron-transfer material by mixing these with a polymer such as polyvinylcarbazole.
Also, use of not only fluorescence but also phosphorescence is being studied with an attempt to elevate the light emission efficiency. If phosphorescence is used, that is, light emission from the triplet excitation state is utilized, an improvement of about three times higher efficiency can be expected as compared with conventional devices using fluorescence (singlet). For this purpose, it has been proposed to form the light-emitting layer using a coumarin derivative or a benzophenone derivative (see, The 51st Autumn Meeting, 1990; The Japan Society of Applied Physics, 28a-PB-7 (1990)). However, only an extremely low luminance could be obtained. Thereafter, use of an europium complex is studied with an attempt to utilize the triplet excitation state, however, high light emission efficiency could not be attained either by this use.
Recently, it has been reported that red light emission with high efficiency can be attained by using a platinum complex (T-1) shown below (see, Nature, Vol. 395, page 151 (1998)). Thereafter, the efficiency in green light emission is further greatly improved by doping an iridium complex (T-2) shown below to the light-emitting layer (see, Appl. Phys. Lett., Vol. 75, page 4 (1999)). 
For applying the organic electroluminescent device as a display device such as flat panel display, further, as a light source for fluorescent lamp or marker lamp, the light emission efficiency of the device must be more improved.
The organic electroluminescent device using the phosphorescence molecule (T-2) described in the above-described publication emits light with relatively high efficiency, however, the organic electroluminescent device using (T-1) is low in the light emission efficiency as compared with devices using (T-2). The main cause of this is presumed to reside in the relationship between the host material in the light-emitting layer and the phosphorescent substance.
The formation probability of triplet exciton is three times higher than that of singlet exciton and therefore, if light emission from the triplet exciton (namely, phosphorescence) is used, the light emission efficiency is elevated. However, use of a phosphorescent substance alone suffers from bad stability of film and low mobility of electric charge (hole or electron) injected from electrodes and accordingly, the light emission efficiency is not elevated. On the other hand, use of a host material alone cannot provide light emission from a triplet exciton and the material uses its energy mostly for heat and is deactivated, as a result, the light emission efficiency is not elevated. For overcoming this problem, a method of forming a light-emitting layer by dispersing a phosphorescent substance in a host material which exhibits fluorescent property is employed.
According to this method, triplet excitons generated from the host material are used as triplet excitons for the phosphorescent substance to cause light emission therefrom. However, this method involves energy transfer and unless the excited triplet level in the host material is close to the excited triplet level of the phosphorescent substance, the probability of energy transfer decreases and the triplet excitons cannot contribute to the light emission. In the case of the above-described device using (T-1) and (T-2), the excited triplet level of the host material used is considered close to the excited triplet level of (T-2).
Under these circumstances, the present inventors have made investigations on the method for, in an organic electroluminescent device using a phosphorescent emission, attaining high-luminance and high-efficiency light emission from a phosphorescent material which by itself does not emit light with high efficiency.
In order to apply an organic electroluminescent device to a display device such as flat panel display, the device must be ensured with sufficiently high stability at the driving in addition to the improvement of light emission efficiency. However, the organic electroluminescent device using the phosphorescence molecule (T-2) described in the above-described publication, which ensures high-efficiency light emission, is insufficient in the driving stability for practical use (see, Jpn. J. Appl. Phys., Vol. 38, page L1502 (1999)). Thus, a high-efficiency display device cannot be realized.
This driving deterioration is presumed to arise mainly because of deterioration of the light-emitting layer.
Electric charges injected from electrodes form electron-hole pairs (excitons) at a certain probability. In general, the light emission (phosphorescence) by triplet excitons has a longer lifetime as compared with the light emission (fluorescence) by singlet excitons but, on the contrary, the singlet exciton has higher thermal stability than triplet exciton.
With increase of a current applied to a device, electric charges injected to the light-emitting layer increase and accompanying this, the amount of electric charges not participating in the generation of excitons also increase. Also, the amount of excitons which are generated but not contribute to the light emission in the light-emitting layer and are thermally deactivated increases and this causes elevation of the temperature of the light-emitting layer.
At this time, the device is considered to deteriorate because the triple exciton is inferior in the thermal stability as compared with singlet exciton. This can be also verified from the fact that the light emission efficiency of the organic electroluminescent device using the phosphorescence molecule (T-2) greatly decreases with the increase of injected current (see, Appl. Phys. Lett., Vol. 75, page 4 (1999)).
As such, organic electroluminescent devices using a phosphorescence molecule have a serious problem in the device driving stability for the practical use at present.
For applying an organic electroluminescent device to a display device such as flat panel display, polychromatic display must be realized. In recent years, compact display devices such as portable telephone, which are a promising use of the organic electroluminescent device, are also required to have polychromatic display.
For realizing polychromatic display such as multicolor display and full color display by using an organic electroluminescent device, the following methods have been heretofore proposed:                1) a method of providing light emission sub pixel for desired colors such as red (R), green (G) and blue (B),        2) a method of disposing a color filter on a white light-emission layer and coloring the emitted light, and        3) a method of disposing a fluorescence converting layer on a blue light-emitting layer and converting the color of emitted light.        
Among these, the method 1) uses no layer of absorbing emitted light, such as color filter, and ensures high use efficiency of light and therefore, this method is ideal for high-efficiency self-light emission-type polychromatic display devices. However, in this method, the materials suitable for respective emitted light colors must be prepared.
For this purpose, as described above, for example, a fluorescence dye for laser, such as coumarin, is doped into an aluminum complex of 8-hydroxyquinoline used as a host material to convert the light emission wavelength (see, J. Appl. Phys., Vol. 65, page 3610 (1989)).
However, since conventionally proposed methods of doping a fluorescence dye all utilize the light emission by singlet excitons, the probability of generating excitons is theoretically low as described above and a sufficiently high light emission efficiency cannot be attained. Furthermore, the method of using a phosphorescence molecule such as (T-1) or (T-2) has a problem in that the number of molecules known to emit phosphorescence at room temperature is very small and desired colors cannot be afforded.
By taking account of these circumstances, the present inventors have studied on the method for attaining high efficiency, high driving stability and capability of polychromatic display, and succeeded in reaching the present invention.